In internal engines such as automobile engines, engine oil and oil additives are deteriorated or degraded by oxidation and heat depending on the engine operation state. The oxidized oil tends to absorb moisture from ambient air, and oil decomposed by heat is thermally polymerized by engine heat to have larger molecular weights. It is known that oil is deteriorated by the inclusion of moisture, dilution with a fuel, the nitration of unburned fuel, etc. The oil is finally turned to muddy deposit called “oil sludge,” which is adhered as solid deposit to engine parts. The solid deposit acts to wear parts and clog oil paths, further causing parts to stick to each other at worst, crippling their functions.
FIG. 3 shows a coil-spring-loaded oil control ring 100 received in a ring groove 91 of a piston 9. A circular oil ring body 200 having a gap is constituted by a pair of axially arranged upper and lower rails 110, 110, and a web 120 connecting them. The coil-spring-loaded oil control ring 100 comprises the above oil ring body 200, and a coil expander 300 pushing the oil ring body 200 radially outward, having a so-called oil-controlling function of keeping a proper amount of oil (within its minimum necessity) on a cylinder wall. In such oil control ring 100, oil sludge is adhered to and accumulated on a surface of the coil expander 300 and an inner circumferential groove 130 of the oil ring body 200, and further to oil holes 140 and an outer circumferential groove 150, likely clogging the oil holes 140. The clogged oil holes 140 fail to exhibit an oil-controlling function, resulting in increased oil consumption. Also, when the oil sludge is adhered and accumulated in coil pitch of the coil expander 300, adjacent coil wires likely stick to each other, losing tension. Particularly, when the coil expander 300 has low tension to improve fuel efficiency, the coil expander 300 becomes unmovable by oil sludge adhered and accumulated in coil pitch, losing a force of pushing the oil ring body 200, and resulting in lower followability of the oil control ring 100 along the cylinder wall.
FIG. 4 shows an expander/segment oil control ring 500 received in a ring groove 91 of a piston 9. The expander/segment oil control ring 500 comprises a pair of circular side rails 600, 600 each having a gap, and a spacer expander 700 supporting the side rails 600, 600, an angled ear section 160 of the spacer expander 700 pushing the side rails 600 in both radial and axial directions to exhibit a sealing function between a cylinder wall surface and the upper and lower surfaces of the ring groove in addition to the above oil-controlling function. Particularly, because a small-width expander/segment oil control ring 500 with a small axial width, namely a reduced size h1, has good followability to a cylinder wall surface as well as the above side-sealing function, it has low friction loss even at a low tension without increasing oil consumption. However, even this expander/segment oil control ring 500 likely suffers the adhesion and accumulation of oil sludge particularly in each space 180 between the ear section 160 and outside flat portion 170 of the spacer expander 700 and the side rails 600. Particularly when the expander/segment oil control ring 500 is made smaller in width, oil sludge tends to be accumulated, having the side rails 600 stick to the spacer expander 700. As a result, the side rails 600 have less followability to the inner surface of the cylinder, resulting in larger oil consumption.
As a method for preventing the adhesion and accumulation of oil sludge to engine parts such as oil control rings described above, pistons, etc., a oil repellent treatment has conventionally been investigated. This treatment forms an oil-repellent coating on engine part surfaces to prevent oil sludge in engine oil from adhering to them. Materials used in the oil repellent treatment are mostly fluorine-containing materials including polytetrafluoroethylene, fluoroalkyl silanes, etc. For example, JP 7-246365 A proposes a sol-gel method for forming an oil-repellent film from metal alkoxides and fluoroalkyl-substituted metal alkoxides in which part of alkoxyl groups are substituted by fluoroalkyl groups. It is known that fluoroalkyl-containing materials have water and oil repellency, and the existence of a fluoroalkyl group on the coating surface provides engine parts with oil repellency, preventing the adhesion and accumulation of oil sludge.
However, JP 10-157013 A describes that the coatings of JP 7-246365 A formed by a sol-gel method using fluoroalkyl-substituted metal alkoxides are extremely thin, not suitable for practical use. Thus, JP 10-157013 A and JP 2000-27995 A propose methods of polymerizing fluoroalkyl-substituted alkoxides before applying coating solutions to substrates, thereby providing thicker coatings.
As described above, conventionally investigated methods for preventing the sticking of engine parts due to the adhesion and accumulation of oil sludge are the volatile oil treatments of engine part surfaces. It has been found, however, that conventional oil-repellent coatings fail to sufficiently prevent the adhesion of oil sludge at high temperatures. Because engine oil exposed to high temperatures in an engine during operation has different properties and behavior from those at room temperature, oil sludge partially adhered to engine parts in a high-temperature operation is further heated in a high-speed operation, so that it is solidified on the engine part surfaces, causing the wearing of parts, the sticking of piston rings, etc. Thus, engine-parts-coating compositions capable of preventing the adhesion and accumulation of oil sludge for a long period of time, and engine parts having such coatings have not been materialized yet.